This invention relates to a drive assembly for tractors for use in agriculture and civil engineering, and more particularly to improvements in the front wheel drive assembly of four-wheel drive tractors.
For use in agriculture and civil engineering, four-wheel drive tractors are already known in which all the front and rear wheels are adapted to be driven. Four-wheel drive tractors have the advantage of giving great traction utilizing the overall weight of the tractor and possessing an increased ability to ascend an upward slope. However, since the front wheels of tractors are used for steering, the front wheel drive assembly may possibly impose restrictions on the steering function of the front wheels, failing to give a large steering angle. For agricultural uses, tractors are operated under special conditions, so that the front drive assembly involves problems. The differential unit, which is disposed at an approximate midportion between the front wheels, is positioned at a relatively low level and therefore entails difficulties when the tractor is driven over ridges and furrows. The operation on the paddy field requires a completely sealed drive assembly, but a sealed structure is difficult to provide for the front wheels which must be steerable.
These difficulties can be substantially overcome when a bevel gear system is employed as the terminal reduction gear means for the front wheel drive unit. However, if it is attempted to use a large tire as the front wheel for a front wheel axle which is positioned at low level, the vertical distance between the front wheel hub shaft and the front wheel axle becomes too large, consequently necessitating the use of a small tire for the front wheel. It would then be difficult to fully ensure the advantage of the four-wheel drive tractor. Accordingly when it is desired to use a large tire for a low wheel shaft, there arises a need to employ spur gear means to deliver the power to an upper transmission unit from which the power is transmitted to the terminal reduction gear means of the bevel gear type. This not only makes the drive assembly complex but also causes the gear housing or the like to limit the steering movement of the front wheel, reducing the steering angle of the wheel. Moreover the heavy structure requires an increased steering force and lowers the performance.